The Galactic Thick Disk Stellar Abundances

By J. Xavier Prochaska, Sergei O. Naumov, B. Carney, A. McWilliam, A. Wolfe. 2000

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We present first results from a program to measure the chemical abundances of a large (N > 30) sample of thick disk stars with the principal goal of investigating the formation history of the Galactic thick disk. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Keck I telescope. Our analysis confirms previous studies of O and Mg in the thick disk stars, which reported enhancements in excess of the thin disk population. Furthermore, the observations of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the exception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] ≈ -1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. The principal results of our analysis are as follows. (1) All 10 stars exhibit enhanced α/Fe ratios with O, Si, and Ca showing tentative trends of decreasing overabundances with increasing [Fe/H]. In contrast, the Mg and Ti enhancements are constant. (2) The light elements Na and Al are enhanced in these stars. (3) With the exception of Ni, Cr, and possibly Cu, the iron-peak elements show significant departures from the solar abundances. The stars are deficient in Mn, but overabundant in V, Co, Sc, and Zn. (4) The heavy elements Ba and Y are consistent with solar abundances, but Eu is significantly enhanced. If the trends of decreasing O, Si, and Ca with increasing [Fe/H] are explained by the onset of Type Ia SN, then the thick disk stars formed over the course of ≳1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scenarios for the formation of the thick disk. Models which consider the heating of an initial thin disk—either through "gradual" heating mechanisms or a sudden merger event—are favored. These observations provide new tests of theories of nucleosynthesis in the early universe. In particular, the enhancements of Sc, V, Co, and Zn may imply overproduction during an enhanced α-rich freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting trends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current understanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent with the ages (tage > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretations of the abundance patterns of the damped Lyα systems. The observations of mildly enhanced Zn/Fe imply an interpretation for the damped systems which includes a dust depletion pattern on top of a Type II SN enrichment pattern. We also argue that the S/Zn ratio is not a good indicator of nucleosynthetic processes.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars (2003, background): “Recently, the chemical evolution of the stellar population in the thick disk has become a subject of intense study and discussion (e.g. Mashonkina & Gehren 2001; Tautvaišienė et al. 2001; Chen et al. 2000; Gratton et al. 2000; Prochaska et al. 2000; Fuhrmann 1998).”
alpha-, r-, and s-process element trends in the Galactic thin and thick disks (2004, background): “2001; Mashonkina & Gehren 2001; Gratton et al. 2000; Prochaska et al. 2000; Chen et al. 2000; Fuhrmann 1998).”
A Structural and Dynamical Study of Late-Type, Edge-on Galaxies. II. Vertical Color Gradients and the Detection of Ubiquitous Thick Disks (2002, background): “Similar differences are seen between the thin and thick disks of the Milky Way, in addition to significant metallicity and relative abundance differences (Tautvaiŝiene et al. 2001; Prochaska et al. 2000; Gratton et al. 2000; Furhmann 1998).”
THE STELLAR POPULATION STRUCTURE OF THE GALACTIC DISK (2015, background): “…& Reid 1983; Jurić et al. 2008; Rix & Bovy 2013), with stars in the thicker of the two components being older, more metal-poor, and [α/Fe]enhanced with respect to stars in the thinner components (Fuhrmann 1998; Prochaska et al. 2000; Bensby et al. 2005; Reddy et al. 2006; Haywood et al. 2013).”
THE GENESIS OF THE MILKY WAY'S THICK DISK VIA STELLAR MIGRATION (2010, background): “…RCUK Fellow 4 University of Zagreb, Croatia 5 Department of Physics and Astronomy, McMaster Univer- sity, Hamilton, Ontario, L8S 4M1, Canada 1998; Prochaska et al. 2000; Tautvaǐsienė et al. 2001; Bensby et al. 2003; Feltzing et al. 2003; Mishenina et al. 2004; Brewer 2004; Bensby et al.…”
THE CHEMICAL EVOLUTION OF THE DRACO DWARF SPHEROIDAL GALAXY (2009, methodology): “Corrections for hyperfine structure for Sc II, V I, Mn I, Co I, Cu I, Ba II, and Eu II, when necessary, were used; the majority of the HFS patterns were adopted from Prochaska et al (2000).”
Impacts of a flaring star-forming disc and stellar radial mixing on the vertical metallicity gradient (2016, background): “…spectra of kinematically selected nearby geometrically thick disc stars show systematically higher α-element abundance with respect to the iron abundance, [α/Fe], compared to kinematically colder geometrically thin disc stars with similar [Fe/H] (e.g. Prochaska et al. 2000; Feltzing et al. 2003).”
Searching for Planets in the Hyades. IV. Differential Abundance Analysis of Hyades Dwarfs (2003, background): “We note that Prochaska et al. (2000) also see inconsistencies between these damping enhancements.”
Simulations of Galaxy Formation in a Λ Cold Dark Matter Universe. II. The Fine Structure of Simulated Galactic Disks (2002, background): “Although varying in importance from galaxy to galaxy, the thick disk appears to be old (∼>10-12 Gyr in the case of the Milky Way, Gilmore, Wyse & Jones 1995), poor in metals (Prochaska et al 2000), and of similar radial extent as the thin disk component (Wyse 2000).”
Stellar Chemical Signatures and Hierarchical Galaxy Formation (2004, background): “…old thin disk during a merger is also not ruled out; however, the Galaxy’s thick disk itself cannot be comprised of the remnants from a lowmass (dSph) dwarf galaxy or from a high-mass dwarf galaxy like the LMC or Sgr, because of differences in chemistry (Bensby et al. 2003; Prochaska et al. 2000).”
Abundances in a Large Sample of Stars in M3 and M13 (2004, background): “The corresponding hyperfine structure constants were taken from Prochaska et al. (2000).”
Dissecting simulated disc galaxies - I. The structure of mono-age populations (2014, background): “…a larger scale-height than the thin disc (Robin et al. 1996; Chen et al. 2001; Jurić et al. 2008), but it is also older, more metal-poor, and enhanced in alpha elements (Gilmore, Wyse, & Jones 1995; Fuhrmann 1998; Prochaska et al. 2000; Bensby et al. 2005; Reddy, Lambert, & Allende Prieto 2006).”
Through thick and thin: kinematic and chemical components in the solar neighbourhood (2010, background): “…older and more highly enriched in α elements than their thin disk counterparts supports the twocomponent nature of the Galactic disk (Gratton et al. 1996; Fuhrmann 1998; Prochaska et al. 2000; Bensby et al. 2003; Gilmore & Wyse 1985; Reddy et al. 2006; Bensby et al. 2007; Fuhrmann 2008).”
Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. III. Abundance Ratios (2001, background): “The corresponding hyperfine structure constants were taken from Prochaska et al. (2000).”
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey (2005, background): “The [Mg/Fe] abundance ra- tio for thick-disk members remains as high as 0.3 dex up to ½Fe/H ’ 0:4 (Fuhrmann 1998; Prochaska et al. 2000; Bensby et al. 2003; Reddy et al. 2003, 2005; Brewer & Carney 2006), and equation (2) would therefore underestimate this ratio by 0.2 dex for such stars.”
THE VERTICAL METALLICITY GRADIENT OF THE MILKY WAY DISK: TRANSITIONS IN [α/Fe] POPULATIONS (2014, background): “…thin disk is metal-rich (mean [Fe/H]≈ −0.2) and α-poor, while the thick disk is relatively metal-poor (mean [Fe/H]≈ −0.6) and enhanced in [α/Fe] (Gilmore & Wyse 1985; Wyse & Gilmore 1995; Chiba & Beers 2000; Fuhrmann 1998, 2011; Prochaska et al. 2000; Bensby et al. 2003, 2005; Reddy et al. 2006).”
The New Galaxy: Signatures of Its Formation (2002, background): “Reconstructing star clusters within the thick disk is a particularly interesting prospect since the disk is likely to have formed within 1−1.5 Ga of the main epoch of baryon dissipation (Prochaska et al. 2000).”
THE SPATIAL STRUCTURE OF MONO-ABUNDANCE SUB-POPULATIONS OF THE MILKY WAY DISK (2011, background): “…et al. 2002; Yoachim & Dalcanton 2005), and metal-poor (compared to the thin-disk components), as well as enhanced in α-elements (Fuhrmann 1998; Prochaska et al. 2000; Tautvaǐsienė et al. 2001; Bensby et al. 2003; Feltzing et al. 2003; Mishenina et al. 2004; Bensby et al. 2005; Reddy et al.…”